This invention is in the general field of large-force reaction type, electro fluid vibrators. More particularly, it is related to vibrators for generating elastic waves in the earth for geophysical purposes. Still more particularly it is concerned with control means to ensure that the vibrator will not provide an alternating force on the earth which is greater than the direct holddown force on the vibrator baseplate in contact with the earth.
In the geophysical industry there are in use large truckmounted electro hydraulic reaction type vibrators. These comprise piston-cylinder systems connected between a baseplate resting on the earth and a large reaction mass. Generally the truck itself, or other vehicle, is used to provide a downwardly direct constant force applied to the baseplate to hold it in contact with the earth. This force is generally applied to the baseplate through a plurality of compliances, which serve to transmit the direct force, while isolating the truck mass from the vibrating baseplate.
If the alternating force generated between the reaction mass (which will be considered to be substantially fixed in space) and the baseplate, is greater than the direct positive force applied by the truck through the compliances to the baseplate, then the baseplate will have intermittent negative forces tending to lift it out of contact with the earth.
This loss of contact is disastrous for two reasons. First, and foremost, it produces a high level of noise and harmonics in the generated seismic waves. Second, it may cause great damage to the vibrator and truck. For these reasons a control means is required to ensure that there will never be a net negative (upwardly directed) force on the baseplate while it is operating.